java整数参考 [英] java Integer reference

问题描述

我有一个问题.

public class Jaba {

    public static void main(String args[]) {
        Integer i = new Integer(0);        
        new A(i);
        System.out.println(i);
        new B(i);
        System.out.println(i);
        int ii = 0;        
        new A(ii);
        System.out.println(ii);
        new B(ii);
        System.out.println(ii);    
    }

}

class A {

    public A(Integer i) { ++i; }

}

class B {

    public B(int i) { ++i; }

}

在我看来，将int \ Integer作为Integer传递给函数并在该引用上进行++应该会更改基础对象，但是在所有情况下输出均为0.为什么会这样?

To my mind passing an int\Integer as Integer to a function and making ++ on that reference should change the underlying object, but the output is 0 in all the cases. Why is that?

推荐答案

如其他答案中所述，Java只执行按值调用，而 ++ 运算符仅影响变量，而不影响对象.如果要模拟按引用调用，则需要传递一个可变对象(如数组)并修改其元素.

As said in the other answers, Java does only call-by-value, and the ++ operator only effects a variable, not an object. If you want to simulate call-by-reference, you would need to pass a mutable object, like an array, and modify its elements.

Java API为此有一些专用对象，例如 java.util.concurrent.atomic.AtomicInteger (它还可以在多个线程上运行)和 org.omg.CORBA.IntHolder (用于CORBA机制的远程调用的按引用调用).

The Java API has some specialized objects for this, like java.util.concurrent.atomic.AtomicInteger (which additionally also works over multiple threads), and org.omg.CORBA.IntHolder (used for call-by-reference for remote calls by the CORBA mechanism).

但是您也可以简单地定义自己的可变整数:

But you can also simply define your own mutable integer:

class MutableInteger {
    public int value;
}


class C {
   public C(int[] i) {
       ++i[0];
   }
}
class D {
   public D(MutableInteger i) {
       ++i.value;
   }
}
class E {
   public E(AtomicInteger i) {
       i.incrementAndGet();
   }
}
public class Jaba {
    public static void main(String args[]) {

        int[] iii = new int[]{ 0 };
        System.out.println(iii[0]);
        new C(iii);
        System.out.println(iii[0]);
        MutableInteger mi = new MutableInteger();
        System.out.println(mi.value);
        new D(mi);
        System.out.println(mi.value);
        MutableInteger ai = new AtomicInteger(0);
        System.out.println(ai);
        new E(ai);
        System.out.println(ai);
    }
}

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